Identification of natural red and purple dyes on textiles by Fiber-optics Reflectance Spectroscopy

Accepted Manuscript Identification of natural red and purple dyes on textiles by Fiberoptics Reflectance Spectroscopy

M.A. Maynez-Rojas, E. Casanova-González, J.L. Ruvalcaba-Sil PII: DOI: Reference:

S1386-1425(17)30107-5 doi: 10.1016/j.saa.2017.02.019 SAA 14941

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

15 November 2016 7 February 2017 8 February 2017

Please cite this article as: M.A. Maynez-Rojas, E. Casanova-González, J.L. RuvalcabaSil , Identification of natural red and purple dyes on textiles by Fiber-optics Reflectance Spectroscopy. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi: 10.1016/j.saa.2017.02.019

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ACCEPTED MANUSCRIPT Identification of natural red and purple dyes on textiles by Fiber-optics Reflectance Spectroscopy

M.A. Maynez-Rojasa, E. Casanova-Gonzálezb*, J.L. Ruvalcaba-Silc

Ciudad

Universitaria,

Coyoacán,

04510,

Mexico

City,

Mexico.

Email:

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S/N,

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a) Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la Investigación

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[email protected]

b) CONACyT - Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la

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Investigación S/N, Ciudad Universitaria, Coyoacán, 04510, Mexico City, Mexico. Email: [email protected]

S/N,

Ciudad

Universitaria,

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c) Instituto de Física, Universidad Nacional Autónoma de México, Circuito de la Investigación Coyoacán,

Mexico

City,

Mexico.

Email:

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[email protected]

04510,

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* Corresponding author: [email protected]

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Abstract

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Understanding dye chemistry and dye processes is an important issue for studies of cultural heritage collections and science conservation. Fiber Optics Reflectance

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Spectroscopy (FORS) is a powerful technique, which allows preliminary dye identification, causing no damage or mechanical stress on the artworks subjected to analysis. Some information related to specific light scattering and absorption can be obtained in the UV-visible and infrared range (300 – 1400 nm) and it is possible to discriminate the kind of support fiber in the near infrared region (1000 – 2500 nm). The main spectral features of natural dye fibers samples, such as reflection maxima, inflection points and reflection minima, can be used in the differentiation 1

ACCEPTED MANUSCRIPT of various red natural dyes. In this work, a set of dyed references were manufactured following Mexican recipes with red dyes (cochineal and brazilwood) in order to determine the characteristic FORS spectral features of fresh and aged dyed fibers for their identification in historical pieces. Based on these results,

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twenty-nine indigenous textiles belonging to the National Commission for the

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Development of Indigenous People of Mexico were studied. Cochineal and

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mixture of cochineal and indigo for purple color.

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brazilwood were successfully identified by FORS in several pieces, as well as the

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Keywords

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FORS, red dyes, non-invasive analysis, Mexican textiles

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1. Introduction

Dyeing has been an important activity in many cultures throughout history.

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Similar techniques for dye extraction and application can be found at different

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epochs and geographical locations. Many of such dyeing methods remained in use for centuries, and some have survived until today.

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From the second half of the 16th century, American cochineal became one of the main dyes in industry. The contribution of the insect to European dyeing is illustrated by two sets of documents: the notes kept by dyers Dirc van der Heyden of Delft, in Holland, in 1631 and Antoine Janot of Saint-Chinian in 1744 [1]. The marketing of cochineal was a successful economic activity due to its huge demand in Europe. Among the goods imported from the New World, only precious metals provided bigger benefits for the Spanish Crown than cochineal [2]. Cochineal 2

ACCEPTED MANUSCRIPT market remained a very profitable activity until the invention of synthetic dyes in the nineteenth century. In Asia and Europe redwoods were regularly used for dyeing; the list of plants includes Caesalpinia sappan (known as patanga in Hindi or suwo in

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Japanese), Pterocarpus santalinus L. (red sandalwood) and Pterocarpus indicus

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(narrawood, Andaman redwood or padauk), among others [1]. The discovery of the

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New World was also the discovery of a whole new universe of plants. Since various tree species with redwoods were exploited in different regions of Central

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and South America, their commercial names were often associated with their place

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of origin. It is not always simple or possible to be sure which botanical species were meant by brazilwood, Campeche wood or Nicaragua wood. Some of them,

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like brazilwood, belong to Caesalpinia sp., while in Mexico and the Caribbean the

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common redwoods are Haematoxylum sp., such as Campeche wood [1,3]. Redwood trees were already a source of dyes in pre-Hispanic times in

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Mexico. In the Maya language, the name of the tree is “ek” and natives used it to

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tint some threads to braid their hairs and to dye some clothes [1]. It is not clear whether this is the same tree as that the Aztecs called huitzquahuitl. Juan Badiano

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and Martin de la Cruz in their Aztec Herbal of 1522 described the decoction of wood as giving a red color [4]. Also Bernardino de Sahagún in Historia General de las cosas de Nueva España [5] dedicated a whole chapter to dyes, among which are redwoods. After the conquest, America became the principal supplier of redwood for the Spanish and Portuguese kingdoms [1]. In Mexico, both cochineal and brazilwood are still used by Mayan, Mixtec and Otomi communities in textile manufacturing and lacquerware production. The 3

ACCEPTED MANUSCRIPT identification of the dyes present in heritage objects is an important step in understanding manufacturing processes and preserving local traditions, while providing key information for the conservation and restoration of such objects. However, there are several factors that hamper the detection of the dye in artistic

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or historical objects. Usually, due to its high tinting power, the dye is present in a

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very low concentration and it can also be mixed with one or more dyes, salts or

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other substances, to yield a different hue.

Furthermore, since the objects may have been exposed to unknown levels

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of light, humidity and temperature for tens to hundreds of years, the dye molecules

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structure may have been transformed by environmental exposure conditions and aging. Hence, the analytical technique applied to its identification must be sensitive

performance

liquid

chromatography

(HPLC)

[6,7]

and

gas

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High

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and specific enough to account for those restrictive factors.

chromatography (GC) [8] are, by far, the most common analytical techniques

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reported in literature for the characterization of dyed textiles. Serrano [6] has tested

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a soft extraction method combined with a new dye recovery treatment, for historical cochineal-dyed European textiles. Due to its mildness, the method yielded good

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results in the identification of minor components. In American textiles, J. Wouters et al. [7] studied a series of Peruvian pre-Columbian and present day textiles using HPLC. They were able to identify cochineal, along with purpurin, relbunium, indigotin, tannins, luteolin and other dyes. Yet, HPLC requires sampling, which is not always allowed in cultural heritage analysis. An alternative is the use of Raman and Surface-Enhanced Raman spectroscopies, either by direct application of the nanoparticles on the fiber [9,10] 4

ACCEPTED MANUSCRIPT of by the generation of the SERS substrate in situ [11]. Still, sampling remains a requisite in most cases, even if a single dyed fiber may be sufficient for the analysis. Most research is focused on European artworks, while cultural objects of American origin have been the subject of only a few studies of this type [7,10,12].

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Useful preliminary information can be derived from portable, non-invasive

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techniques that allow in situ examination. The non-invasive approach for the study

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of historical and archaeological artworks typically employs a set of spectroscopic techniques [13]. Among those techniques, Fiber Optic Reflectance Spectroscpy

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(FORS) may provide a fast, portable and non-invasive methodology for dyes and

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pigments identification [14].

UV-Visible-Near infrared FORS is an optical spectroscopy, which is able to

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detect electronic transitions in the UV-visible range and vibrational overtones and

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combinations in the near infrared region. The technique offers many advantages, including the lack of sampling and its high sensitivity for the identification of dyes.

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FORS has been applied to the identification of artistic materials used in illuminated

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manuscripts [15], wall paintings [16,17], and occasionally to the analysis of dyes coupled with inorganic substrates to make lake pigments [14,18,19].

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FORS has been used to investigate the material employed in the production of artworks, in particular inorganic pigments but also painting lakes and modern artistic materials. FORS is much less applied to the study of colored textiles [20,21]. Gulmini et al. prepared eleven different natural dye references (red, yellow, blue, green and purple colors) and used the gathered information for the analysis of an historical cloth from a church in Bosco Marengo near Alessandria, Italy. They reported a great homogeneity throughout the cloth; red and pink areas provided 5

ACCEPTED MANUSCRIPT reproducible spectra associated to cochineal, whereas for the blue and green areas they suggested the use of indigo or woad. The spectroscopic approach was not sufficient for the identification of yellow dyes, where the use of HPLC was required. Previously, Angelini et al. analyzed by FORS references textiles dyes and

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19th century rugs from Azerbaijan and Turkey. FORS identification of yellow dyes

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proved difficult, but for the red areas it was reported the use of insect dyes, most

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probably cochineal. Finally, indigo was detected in the blue areas of the rugs. In this paper, we focused in the fabrication of red and purple dyed cotton

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and wool fibers, using traditional Mexican recipes and their analysis using FORS.

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The fibers were dyed with cochineal, brazilwood and indigo in order to get red and purple colors. Afterwards, the technique was applied to the analysis of a set of 19th

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and 20th century textiles, representing the cultural diversity of Mexico and

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safeguarded by the National Commission for the Development of Indigenous People (CDI, Comisión para el Desarrollo de los pueblos Indígenas in Spanish).

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The CDI collection, with over 13000 objects, includes a large selection of Mexico´s

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indigenous textiles and artifacts from all around the country and was founded as part of National Institute of Indigenous People (today CDI) in 1951. The collection

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has played an important role in the dissemination of the cultural diversity of indigenous Mexico.

2. Experimental section 2.1. Sample preparation 2.1.1. Dyeing

6

ACCEPTED MANUSCRIPT Cotton and wool fibers were dyed with cochineal and brazilwood following traditional Mexican recipes [22]. Cochineal (Dactylopius coccus) was bought from local producers in Hidalgo, Central Mexico, in the form of sundried insects. Pieces of brazilwood’s tree bark (Caesalpina echinata), cotton and wool were acquired

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from local suppliers at Mexico City. Raw indigo (Indigofera sp.) was purchased at

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the Textile Museum in Oaxaca, Mexico.

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The fibers were washed for an hour on a bath of boiling water containing flakes of neutral soap and then rinsed with clean water until no more foam is

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formed. The wet fibers were weighed and 500 g of each were poured in a 20 L

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solution containing 130 g of alum (KAl(SO4)2·12H2O) and 30 g of tartaric acid in water. The resulting mixture was boiled for one hour, the excess of water was

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eliminated and the mordanted fibers were left to dry at room temperature.

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Brazilwood tree barks were crushed, poured on 20 L of boiling water and left there for one hour, in order to achieve the extraction of the dye. Cochineal insects

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were mechanically grinded and the resulting powder was soaked in one liter of

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water for three days at 4 °C. Afterwards, the mixture was diluted with 15 L of water and boiled for one hour.

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Dyestuffs extracts were filtered using a metallic strainer for the brazilwood and a soft cloth for cochineal. The filtered solutions were heated again up to 70 °C and the mordanted fibers were added. The dyeing solutions were kept at 70 °C for one hour, after which the fibers were extracted and cooled to ensure safe handling. The dyed fibers were thoroughly rinsed with running water until no color loss was observed.

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ACCEPTED MANUSCRIPT In order to prepare purple dyed fibers, 36 g of iron sulphate (FeSO4·7H2O) were added to the remaining dyeing baths under heating. The previously cotton and wool dyed red fibers were poured onto the new dyeing baths and the mixtures were heated at 70 °C for one hour. Next, the fibers were rinsed as described

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before.

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A second method for preparing purple fibers involves the dyeing of red fibers

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with an indigo bath. Raw indigo (25 g) was finely grinded and dissolved in 1 L of alkaline water (pH of 14). Approximately 60 g of sodium ditionite were added to the

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solution, until a green color was observed and the solution was left to stand for a

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week. Then it was carefully poured into 15 L of water, pH was adjusted to 9 and heated to 60 °C. The cochineal-dyed fibers were immersed in the bath for under 60

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seconds and then shaken in open air to ensure oxidation of indigo.

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2.1.2. Aging

Dyed fibers were artificially aged for a month using a Q-U-V Accelerated

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Weathering Tester from The Q-Panel Company. Treatment consisted of two daily

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cycles of 8 h with exposition to UV-B (310 nm) radiation at 50 °C and 40% relative

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humidity and 4 h without radiation at 45 °C and 80% relative humidity [23,24].

2.2. Analytical methodology Fiber Optics Reflectance Spectroscopy (FORS) analysis were performed with a portable FieldSpect-4 by ASD Inc., Colorado, USA, equipped with 3 detectors working at different spectral ranges: visible, near infrared (NIR) and short wave near infrared (SWNIR). For analysis purposes, the UV-visible and NIR regions are presented together in a zone named UV-visible-near infrared (UVNIR). 8

ACCEPTED MANUSCRIPT UVNIR ranges from 300 nm to 1000 nm with 3 nm as spectral resolution, and SWNIR ranges from 1000 nm to 2500 nm with a 10 nm spectral resolution. Data was acquired via a handheld probe, with an analysis area of 1 cm 2, placed in close contact with the sample. The probe is equipped with a D65

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illuminant, which emulates sunlight. Spectra were recorded with a 0.2 s integration

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time in both reflectance and absorbance (Log [1/R]) modes. This equipment was

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calibrated using a certified reflectance standard provided by ASD Inc (AS-02035000CSTM-SRM-990-362) for each set of measurements. For the absorbance

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mode, data is automatically processed according to the Kubelka-Munk theory

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[25,26]. Inflection points in the reflectance mode were determined using the first derivative spectrum, generated for each spectrum using Origin 2015 Software. For

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statistical purposes, at least five spectra were acquired on each reference fiber or

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dyed textile.

FORS spectra were first acquired from red and purple dyed fibers, both

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fresh and aged, in order to identify the characteristic bands for each dye-fiber

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complex. Once the methodology was established, it was applied to the in situ study of 29 dyed indigenous textiles, belonging to the collection of the Comisión Nacional

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para el Desarrollo de los Pueblos Indígenas (CDI, National Commission for the Development of Indigenous People). Colorimetry measurements were performed with a RUBY non-contact spectrocolorimeter made by STIL, Aix-en-Provence, France with a 4 mm spot size and an 80 mm working distance. The spectral range is 400 to 800 nm and the results are measured in the CIE L*a*b* color space, using a Tungsten halogen lamp as iluminant. Colorimetric measurements were performed after a device 9

ACCEPTED MANUSCRIPT calibration using a lambertian certified reflectance standard (USRS-99-010AS01158-060) by Labsphere, USA. For each sample, five measurements were averaged to obtain the colorimetric parameters.

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3. Results and discussion

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3.1. Fresh dyed fibers

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AN

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cochineal and brazilwood, are shown in figure 1.

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Fresh and aged cotton and wool fibers, dyed with various recipes of

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Figure 1. Dyed fibers. A) cochineal cotton, B) cochineal wool, C) brazilwood cotton, D) brazilwood wool, E) cochineal + FeSO4 cotton, F) cochineal + FeSO4 wool, G) brazilwood + FeSO4 cotton, H) brazilwood + FeSO4 wool and I) cochineal + indigo wool.

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The main coloring agent in cochineal is carminic acid, an anthraquinone,

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while for brazilwood is the isoflavonoid brazilin (figure 2). Typically, the reflectance spectrum of red colorants is characterized by a sigmoidal curve behavior in the visible range and a part of NIR. This spectral region gives information related to the electronic transitions of the molecules in the dye, some features in the reflectance/absorbance

spectra

(maximum

characteristics from each dye.

10

and

inflections

points)

are

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ACCEPTED MANUSCRIPT

Figure 2. Molecular structure of A) carminic acid and B) brazilin

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Carminic acid in neutral solution has an absorption maximum in the visible range at 490 nm, reported by Favaro et al. [27]. Changes in pH or the presence of

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cationic species may induce a bathochromic shift on this band. We found

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maximum bands in the absorption mode in 524 nm and 563 nm for the cotton yarns, while for the dyed wool we found the maxima in 527 nm and 566 nm, as

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shown in figure 3B – D. For dyed wool yarns, Gulmini [20] reported for cochineal

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two absorption maxima, in the 525 – 535 nm and 555 – 570 nm ranges, in good agreement with our results. The same authors described for cochineal an evident

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reflection band in the blue region between 415 and 435 nm [20].

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Our reflectance spectrum presented the same feature in the blue region only for cotton, centered in 423 nm. In the case of wool, the dye concentration led to a saturated spectrum in which the characteristic bands of the dye are more difficult to identify. The reflectance inflection points registered in our reference cochineal set are 623 nm for cotton yarn (Figure 3A) and 661 nm for wool (Figure 3C).

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Figure 3. Reflectance and Log(1/R) spectra of fresh dyed fibers. A) – B) cotton; C) – D) wool.

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On the other hand, brazilwood in wool yarns showed a broad intense

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absorption band centered at 520 nm, according to Gulmini et al. [20]; we found a band centered at 480 nm (401 to 451 nm) for cotton dyed with brazilwood and at 516 nm (380 - 570 nm) for wool yarns (see figure 3C). In addition, Gulmini [20] reported the presence of a weak absorption band between 445 and 450 nm, which was not observed in our reference set. The inflection point in our brazilwood dyed fibers was found at 592 nm for cotton and 615 nm for wool yarns. Those values are in good agreement with the

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ACCEPTED MANUSCRIPT 585 - 600 nm range reported previously [20], and it showed less variability than those of the cochineal red dye. Few more reports can be found in literature regarding FORS analysis of brazilwood. In one of them, lake pigment preparation of brazilwood [28] displayed a

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bathochromic shift in the absorption of at least 40 nm towards greater wavelengths,

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in comparison with fibers.

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The addition of FeSO4 to cochineal and brazilwood to obtain a purple hue provoked changes in the reflectance spectra. For the cochineal and iron sulphate

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mixtures, the spectrum has a maximum reflectance band in the visible range at

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about 410 to 435 nm for both fibers, and absorption bands at 525 nm and 565 nm, for cotton and wool fibers, respectively (figure 3). These bands are in good

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agreement with the findings of Gulmini [20]. However, the inflection points,

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determined by the first derivative of the reflectance, are shifted towards longer wavelengths, 680 nm for cotton and 685 nm for wool.

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In the case of the purple hue generated with brazilin and iron ion, the cotton

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spectrum presented a bathochromic shift with respect to the red dyed fiber and the absorption maximum is observed as a broad band from 400 to 620 nm, centered at

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518 nm. For the spectra of dyed wool fibers, the absorption maximum is centered at around the same value, 517 nm, but the band is much more broadened for the brazilwood and FeSO4 mixture (380 to 670 nm). In all cases, the limits of the absorption bands were difficult to determine. This shift was not observed on the previous report of Gulmini [20], where the addition of mordants did not change the absorption maxima of the dyes.

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ACCEPTED MANUSCRIPT The shift of the inflection point was also observed for brazilwood with iron, as was previously observed for the cochineal and iron mixture. The inflection point in the reflectance spectra moved to greater wavelengths from 592 nm in red brazilwood to 680 nm in the purple brazilwood for cotton, while for wool the

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inflection point shifted from 615 nm in the red fiber to 685 nm in the purple hue.

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For the cochineal and indigo mixture the spectra presented the

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characteristic bands of both dyes. In cotton, the 529 and 569 nm bands, corresponding to cochineal, were observed together with the 649 nm band

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corresponding to the indigo maximum absorption. While in wool, 527, 562, 648 nm

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were the maxima of the absorption bands. These results are similar to what was found in literature [20], but in our case the cochineal absorption bands are more

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evident.

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In the NIR region, all spectra showed a second-order vibrational overtone, assigned to -C-H bonds in both cotton and wool [29]. The bands are located at

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around 1218 nm for cotton and 1187 nm for wool, and are not related to the dyeing

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molecule (see figure 3 A and C). Thus, the presence of such bands on artworks may serve as a marker of the cellulosic or proteinic origin of the fiber used.

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However, to confirm the previous hypotheses, more research is needed involving a wider range of raw fibers like henequen, linen, yute, silk and alpaca, etc. When compared with the fibers dyed with pure cochineal or brazilwood with those dyed with the mixture of dye and iron sulphate (figure 3 A and C), a distinct behavior can be observed in the 700 – 1100 nm region in all spectra. Spectra of cotton and wool dyed with cochineal (or brazilwwod) and FeSO4 showed a less steep slope in that region and a less intense reflectance. The spectra of the fibers 14

ACCEPTED MANUSCRIPT dyed with the mixture of cochineal and indigo were more similar to those dyed with pure colorants in the 750 – 1100 nm region, but a clear difference can be observed from 600 to 750 nm, due to the presence of indigo. The determination of these spectral features, along with the characteristic

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absorption maxima in the visible region, can be useful for the application of

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Hyperspectral Imaging (HSI) of dyed textiles [30]. Thus, the information of a more

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localized technique like FORS can be used as a guide for a more global analysis and would allow the identification of both the support and the dye in extensive

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areas.

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When cotton is dyed, the red or purple hue is less intense than in the wool yarns (figure 2). The wool α-helix proteins structure has a large number of different

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heteroatoms (O, N, S) in which Al3+ can form bonds while only oxygen is available

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in cotton. As a result, less dye molecules are trapped by the cellulosic chain than

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by the protein structure of wool.

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3.2. Aged dyed fibers

The few reports we have found on FORS analysis of dyed fibers are focused

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on the characterization of freshly dyed textiles. Studies dealing with controlled aging processes are mainly performed by HPLC techniques. However, aging is a key factor when trying to identify dyes on historic – and even relatively new – textiles, since the structural changes induced by the natural aging process on the dye molecule may lead to changes in the resulting spectra. In order to take into account this fact, and to gather a set of reference spectra of aged dyes and fibers, the cotton and wool fibers dyed with different recipes of cochineal and brazilwood 15

ACCEPTED MANUSCRIPT were subjected to an accelerated aging process. Aging experiments could not be performed on the cotton fiber dyed with the mix of cochineal and indigo, as the small amount of sample available made it difficult to keep a set of control and aged samples.

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Colorimetric measurements provide a preliminary aging characterization.

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Changes in L*, a* and b* coordinates are related to differences in luminosity, red

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saturation and yellowing, respectively. Table 1 shows the shift of colorimetric parameters after aging. The luminosity increases for all the fibers (positive ΔL*), in

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accordance with the loss of the dye that can be appreciated on figure 1 and with

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the results of Serrano [31]. This increment of the luminosity is more pronounced for the cotton dyed fibers.

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Table 1. Changes in L*, a* and b* coordinates of the dyed fibers before and after accelerated aging.

Dye

ΔL* 15.9 1.6 14.5 10.4 17.5 3.8 23.5 11.6 9.1

Δa* -26.4 -8.2 -17.5 -18.9 -9.6 2.8 -5.5 -9.6 -2.3

Δb* 7.5 1.0 -3.2 0.2 12.9 4.8 8.0 6.1 7.6

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Cochineal Cochineal Brazilwood Brazilwood Cochineal + FeSO4 Cochineal + FeSO4 Brazilwood + FeSO4 Brazilwood + FeSO4 Cochineal + Indigo

Fiber Cotton Wool Cotton Wool Cotton Wool Cotton Wool Wool

Conversely, Δa* values diminished for all the dyed fibers but one (wool dyed with cochineal and iron sulphate), as a result of a substantial loss of saturation of the red hue. More than half of the fibers experienced a yellowing effect after aging. However, Δb* changes are positive but small for wool dyed with cochineal and brazilwood, and negative for cotton dyed with brazilwood. In general, the changes

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ACCEPTED MANUSCRIPT in colorimetric parameters were more acute for the cotton fibers (table 1 and figure 1). Wool and silk aging were the subject of a number of previous studies, including pure [32,33] and dyed fibers [34,35]. HPLC and GC-MS have been

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intensively used to identify the degradation products of both fibers and dyes. In all

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cases, the yellowing of the fibers was attributed to the occurrence of derivatives of

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benzoic acid. In addition, Serrano [31] reported the presence of aspartic, glutamic and cysteine acids - keratin degradation products - as another source of yellow

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hues. Dyes’ degradation products are not so easily identified; Serrano [31] only

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suggested the presence of anthraquinone degradation products. Kirby [36] reported the presence of flavokermesic acid, which may contribute to the yellowish

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color observed on the aged fibers.

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Optical methodologies do not allow identifying the aging degradation subproducts, but some spectral features are clearly observed. Most of the

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characteristics bands in reflectance or absorbance mode can be observed at

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around the same wavelength, but the signals are much less intense and can be difficult to observe (figure 4).

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Cochineal-dyed fibers – both cotton and wool – presented small shifts in the inflection point and absorption maxima before and after aging (table 2). For wool, the absorption bands corresponding to cochineal were slightly more intense in the aged fibers, probably because of the loss of dye reported by Serrano [31], which led to less saturated spectra. Aging effects are more noticeable in the reflectance spectra of brazilwooddyed fibers (figure 4), in particular for cotton, where the spectral features in the 17

ACCEPTED MANUSCRIPT visible range are almost lost. For wool, the broad absorption band centered at 516 nm in the fresh dyed fiber is much less intense and shifted to 542 nm. Inflection

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points, however, remained similar for both fibers (table 2).

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Figure 4. Reflectance and Log(1/R) spectra of aged dyed fibers. A) – B) cotton; C) – D) wool.

Table 2. Reflectance and absorbance maxima (Mx), and inflection point (IP). Fresh Dye fibers

Color

Red

Dye / Fiber

Cochineal / cotton

Reflectance

Aged Dye fibers

Absorbance

Reflectance

Absorbance

Mx

IP

Mx

Mx

Mx

IP

Mx

Mx

423

623

524

563

473

613

537

567

18

ACCEPTED MANUSCRIPT 661

527

566

-

662

520

559

Brazilwood / cotton

-

592

-

480

-

601

-

555

Brazilwood / wool

-

615

-

516

-

608

-

542

Cochineal+FeSO4 / cotton

432

680

525

565

595

685

524

-

Cochineal+FeSO4 / wool

414

685

527

564

600

682

521

559

Brazilwood+FeSO4 / cotton

-

680

-

518

511

573

-

-

Brazilwood+FeSO4 / wool

-

685

-

517

-

587

-

532

Cochineal+Indigo / cotton

433, 612

707

529, 569

649

-

-

-

-

Cochineal+Indigo / wool

431, 540, 608

714

525, 561

460, 547, 614

714

522, 563

649

IP

CR

US 648

T

-

AN

Purple

Cochineal / wool

The analysis of purple fibers yielded mixed results. Cochineal-based dyes

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showed little to no changes in the inflection point and the position of the absorption

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maxima, while the local reflectance maxima in the blue region of the spectra was shifted towards greater wavelengths in all cases, as can be seen on figure 4 and

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table 2. Both cotton and wool purple fibers dyed with brazilwood and iron sulphate

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showed an hypsochromic shift of its inflection point, from 680 nm to 573 nm for cotton and 685 to 587 nm for wool.

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As discussed for the fresh-dyed fibers, overtone vibrational bands corresponding to -C-H vibrations of cotton and wool can be observed in the 1187 – 1220 nm region. The same behavior was observed for the artificially aged fibers. The differences in the slope and intensity observed for the fibers dyed with the mixture of dye and iron sulphate are not so clear for the aged fibers. While for wool (figure 4C) such differences were still present between the fibers dyed with

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ACCEPTED MANUSCRIPT pure brazilwood and the brazilwood + FeSO4 mixture and, in a lesser extent, for the cochineal / cochineal + FeSO4 pair, cotton fibers showed less distinctive features (see figure 4A). This phenomenon may difficult the proposed application of HSI when analyzing dyed cotton textiles that are naturally aged. More research

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is needed in order to clarify this point.

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The correct identification of dyes by fiber optic reflectance spectroscopy

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depends upon a number of factors, including the color saturation and the combination of the values of the inflection point on the reflectance spectrum and

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the absorption maxima of the dye. As seen above, aging can affect these

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parameters. Spectra of heritage objects must be carefully analyzed, in order to take into account the variation in the spectral features induced by the natural aging

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process.

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Red dyes can become difficult to identify when the absorption maxima are too close, as in the case of aged wool fibers dyed with cochineal and brazilwood

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(table 2). However, the general shape of the spectra differs for both dyed fibers, as

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cochineal presents two local absorption maxima, while for brazilwood a local maximum is not easily identified. Instead, brazilwood spectra present a shift of the

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slope in the vicinity of 540 nm. In addition, the inflection points of the reflectance spectra are very different for both dyed fibers (table 2). It is critical to take into account the spectral features of both reflectance and absorption spectra in order to achieve a correct identification of the dye.

3.3. Historical textiles

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ACCEPTED MANUSCRIPT Twenty-nine indigenous textiles, part of the CDI collection, were analyzed by FORS (Table 3). Huipil, serapes, feedbag (morral), dress waistbands and wrap skirts were part of the studied collection, all of them created in different regions of Mexico, many in the 19th century. Most of them were woven using a back-strap

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loom. While wool was the main fiber used, some of the textiles have a mix of wool

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with cotton embroidery.

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The less energetic part of the spectrum, between 1500 and 2500 nm, gives information related with the support fiber. In this range, it is possible to differentiate

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between animal or vegetal origin, thanks to -OH and -NH groups vibrational

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overtones. This region of the spectra has been studied by Delaney in wool and silk fibers by FORS-HSI in historical and archaeological investigations [30] and has

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also been used for fiber identification in textile [37]. Most of our textiles presented

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bands at 1738, 1936, 2055, 2178, 2291 and 2348 nm, related to wool [30]. Sixteen of the textiles showed a good correspondence with cochineal

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reference spectra. Seven of them were identified as brazilwood-dyed yarns and

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only in six cases we could not determine the dye used. For these last six cases, we cannot exclude the use of synthetic dyes, such as anilines, owing to its more

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recent date of manufacturing. However, at the moment we lack of enough information to perform a proper identification of these dyes. Table 3 List of studied textiles and dye identification Textile

Origin

Temporality

Dye identification

Huipil 7254

Tsotsil culture Chiapas, Mexico Tsotsil culture Chamula, Chiapas Mixtec culture Pinotepa de Don Luis, Oaxaca Nahua culture Naupan, Puebla

Mid 19th century

Unidentified

1985

Brazilwood

Mid twentieth century

Unidentified

1970

Cochineal

Huipil 7288 Pozahuaco (skirt) 7314 Belt 7515

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ACCEPTED MANUSCRIPT Cochineal

Early 20th century

Cochineal

Early 20th century

Cochineal

Mid 20th century

Brazilwood

Early 20th century

Brazilwood

Mid 20th century

Cochineal

Canvas 11186

Probably from Saltillo, Coahuila

Early 19th century

Cochineal

Serape 11210

Probably from Saltillo, Coahuila

Late 19th century

Cochineal

Serape 11261

Probably from Saltillo, Coahuila

Cochineal

Morral 14255

Santa María del Río, San Luis Potosí Huichol culture Nayarit Nahua culture Hueyapan, Morelos Otomi culture Probably Temoaya, Mexico State

First half of the 19th century First half of the 20th century Mid of 20th century 1982

Brazilwood

Late 19th century

Mid 20th century

Cochineal Cochineal+Indigo Brazilwood Brazilwood

Early 20th century

Brazilwood

19th century

Cochineal

-

-

Unidentified

Mazahua culture Mexico State Otomi culture Probably Tolimán, Querétaro Tsotsil culture Aldama, Chiapas Probably Saltillo, Coahuila

Mid 20th century

Cochineal

19th century

Cochineal

2002

Unidentified

First half of the 20th century 2009

Cochineal

Overcoat 8007 Serape 8027 Morral 10904

Serape 11003

Pozahuanco (skirt) 14382 Pozahuanco (skirt) 14567

Overcoat 14601

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Sarape 14608

Huipil 16489

Zapotec culture San Pablo Villa de Mitla, Oaxaca Zapotec culture Ocotlán de Morelos, Oaxaca Probably Saltillo, Coahuila

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Serape 14580

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Quechquémitl 14356

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Belt 14292

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Belt 14280

Rebozo 15076

First half of the 20th century -

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Pozahuanco (skirt) 11180

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Morral 7583

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Early 20th century

Pozahuanco (skirt) 11183

Otomi culture Ixmiquilpan, Hidalgo Otomi culture Queretaro Otomi culture Tulancingo de Bravo, Hidalgo Mixtec culture Oaxaca Mazahua culture San Felipe Santiago, Mexico State Otomi culture Xonacatlán, Mexico State Otomi culture Sombrerete, Queretaro -

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Morral 7569

Serape 17222

Pozahuanco (skirt) 18069

Mixtec culture Pinotepa de Don Luis, Oaxaca

Cochineal Brazilwood Cochineal

Cochineal Brazilwood

Unidentified

We will discuss four particular cases from this collection, all consisting of dyed wool and showing the use of pure, mordanted and mixed dyes. The first one is a Serape (Figure 5A) manufactured in Oaxaca by the Mixtec people. Spectra from the red areas are shown in figure 5B-C, displaying a broad absorption band 22

ACCEPTED MANUSCRIPT from 355 to 570 nm, centered at 501 nm. Reflectance spectrum shows a characteristic S shape with inflection point at 602 nm. The spectral features of the textile are similar to those of brazilwood-dyed wool, also shown in figure 5B-C for comparison purposes. The absorption maximum is shifted towards lower

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wavelengths, which could indicate the beginning of an aging process. The CDI

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collection records indicate that the Serape 8027 was manufactured mainly with

wool bands are present on the acquired spectra.

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wool, but a local cellulosic fiber, chichicaxtle, was also employed. However, only

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Figure 6A shows another Serape, probably manufactured in Saltillo,

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Coahuila, Mexico. This textile represents a peyote (Lophophora williamsii) and was made in wool with some cotton yarns. Evidence of the use of cochineal was

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observed on red and pink areas (figure 6B-C). The red hue presents two

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absorption maxima at 524 nm and 555 nm and a reflectance inflection point of 619 nm. The pink color, on the other hand, shows absorption maxima in 521 and 555

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nm and 599 nm as inflection point. Such values are in agreement with our

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reference spectra (figure 3 and 4), discussed above, and with the report from Gulmini [20]. The use of the same dye to generate different hues suggests the

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application of an exhaustion dye method where the less saturated hues are achieved by multiple uses of the same dye bath [22].

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Figure 5. A) Visible light image of Serape 8027. Yellow dots indicate the analyzed areas. B) – C) Reflectance and Log(1/R) spectra of Serape 8027 and red dye references.

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Figure 6. A) Visible light image of Serape 17222. Yellow dots and circles indicate the analyzed areas. B) – C) Reflectance and Log(1/R) spectra of Serape 17222 and red dye references.

Two morrals or bags were studied. The red one is from Santa Maria del Rio in San Luis Potosí (Figure 7A), Mexico and is dated in the first decade of XX century. The bag was made in wool yarns woven in a back-strap loom. Maxima bands in the absorption mode are located in 521 and 555 nm with only one peak in the first derivative at 604 nm (figure 7D). The other morral is from Ixmiquilpan, Hidalgo, Mexico and was manufactured by the Otomi culture (Figure 7B). The 25

ACCEPTED MANUSCRIPT absorption bands appear at 528 nm and 563 nm. In this case the first derivative shows two maxima in the visible range at 607 nm and 750 nm. Both absorption maxima are in the range observed for fresh and aged wool fibers dyed with a mixture of cochineal and iron sulphate. However, for the morral

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14255 (Figure 7A), the inflection point of the reflectance is in the vicinity of the

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ones observed for wool fibers dyed with pure cochineal, while the presence of a

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second inflection point near 750 nm and the shape of the spectrum for the morral 7569 is closer to what was observed for the mixture of cochineal and iron sulphate.

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The precise recipe of dye, mordant and pH conditions employed for dyeing this last

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piece cannot be determined by FORS, unless a large set of references is prepared

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including a wide selection of dyeing recipes.

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Figure 7. A) Visible light image of Morral 14255. B) Visible light image of Morral 7569. Yellow dots indicate the analyzed areas. C) – D) Reflectance and Log(1/R) spectra of Morrals 14255 and 7569, and red dye references.

A fourth textile was created in central Mexico, also by the Otomi culture and was woven in a back-strap loom (Figure 8A). Most of the piece is made of wool, with some cotton details and lateral fringes. The main body of the textile is dyed in 27

ACCEPTED MANUSCRIPT a purple hue, probably a mix of cochineal and indigo. The absorption maxima at 522, 562 and 652 nm are in good agreement with the ones we determined for the wool fibers dyed with a similar mixture (figure 8C). Also, the 728 nm inflection point coincides with the one of indigo.

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Two different reds were studied in the fringes, the darkest one presents an

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inflection point in 605 nm but the absorption maxima are not clearly observed in the

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spectra. Such spectral features suggest the use of brazilwood, as the spectra of these areas of the textile are very similar to those of aged brazilwood-dyed wool

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fibers. On the other hand, the brightest fringes show absorption bands in the range

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of cochineal, at 517 and 552 nm, with an inflection point at 593 nm.

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Figure 8. A) Visible light image of Quechquémitl 14356. Yellow dots indicate the analyzed areas. B) – C) Reflectance and Log(1/R) spectra of Quechquémitl 14356 and dye references.

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ACCEPTED MANUSCRIPT 4. Conclusions Fiber Optics Reflectance Spectroscopy proved useful for the identification and differentiation of red dyes and some of their mixtures. The correct identification of cochineal, brazilwood and three variants was achieved on cotton and wool dyed

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fibers. The effect of aging on the fibers and the dye can be observed on the FORS

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spectra. However, the impact of aging experiments on the identification of the dye

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present on historical textiles was different for cochineal and brazilwood. While the FORS spectra of cochineal on the historical textiles studied was more similar to

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that of the aged dyed fibers, the spectra of brazilwood on Serape 8027 is closer to

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that of the fresh dyed fiber. Further research is needed, including a wider range of

understanding of this point.

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fibers, dyes, dyeing techniques and aging conditions, in order to gain a better

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A comprehensive database of FORS spectra, including several dyes, supports, dyeing recipes and aging periods is essential for a first identification of

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the dyes present on cultural heritage objects. The set of references prepared and

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characterized in this work allowed the identification of the dyes of cochineal and brazilwood used in red areas and cochineal mixed with indigo in purple regions on

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twenty-two out of twenty-nine historical textiles from the collection of the National Commission for the Development of Indigenous People, in Mexico City. The technique permitted a fast, precise and non-invasive in situ characterization of the cultural heritage objects studied, but under the actual experimental conditions FORS presents limitations related to the concentration of the dye, since too intense or too fainted colors produce spectra where the characteristic bands of the dyes cannot be observed. 30

ACCEPTED MANUSCRIPT FORS can provide a powerful tool for the study of valuable textile pieces, without the need of sampling as a first stage. Since a large number of spectra can be recorded in a short period of time, the technique can be used for the screening of large collections, yielding information of the fibers and dyes used and may help

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in determining subsequent analyses including a suitable sampling strategy.

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The combination of punctual analyses by FORS and Hyperspectral Imaging

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for pigment mappings is a very suitable approach, since FORS can be used to determine the specific wavelengths that yield the most valuable information for

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HSI, thus allowing a rapid and precise analysis of large objects. The correct

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identification of the dye present can be further confirmed by spectroscopic techniques, such as Raman, SERS and spectrofluorimetry. In the case of UV-Vis

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spectrofluorimetry, it is essential to develop portable equipment for in situ studies.

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More research is needed, including the generation of spectral databases comprising a large number of red dyes analyzed with a combination of several

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spectroscopic techniques.

Acknowledgements

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This research was carried out at the Laboratorio Nacional de Ciencias para la Investigación y Conservación del Patrimonio Cultural (LANCIC), Instituto de Física, Universidad Nacional Autónoma de México and it has been supported by CONACYT Projects LN 232619, LN 260779, LN 271614 and LN 279740 and for LANCIC instrumentation, as well as CONACYT projects 239609 and 131944 and PAPIIT UNAM IN110416 that funded this research development.

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ACCEPTED MANUSCRIPT Authors wish to thank Isaac Rangel Chavez for the images of the textiles, Elsa M. Arroyo-Lemus and Tatiana Falcón-Álvarez for their support in the selection of the historical textiles. Also to Octavio Murillo Álvarez de la Cadena, Rene López Bedolla and the National Commission for the Development of Indigenous People,

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for their help during the examination of the historic textile collection.

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M. Maynez acknowledges CONACYT for a grant and the Material Science

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and Engineering PhD program of Universidad Nacional Autónoma de México (UNAM).

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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FORS proved useful for the identification and differentiation of red and purple dyes Fast, accurate and non-destructive characterization of textiles without sampling Modern Mexican dyeing recipes were used for generating a set of reference spectra FORS reference spectra are essential for a suitable identification of the dyes

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